With drought, desertification, water environment deterioration, etc., all of which are worsening in recent years, water treatment techniques are more important than ever before, and a separation membrane utilization technique has been widely applied. As for sea water desalination, the development of reverse osmosis technology has increased reliability and reduced costs. Due to the adoption of a desalination process using reverse osmosis membrane, many reverse osmosis desalination plants have been built and are operating in areas with extremely scarce water resources, such as the Middle East, Caribbean Islands, and the Mediterranean.
On the other hand, as a technique for obtaining fresh water from sea water, a process referred to as the membrane distillation method that uses heat as a driving force to obtain fresh water via a membrane as with an evaporation method, has been proposed and is under consideration.
The membrane distillation method is, in general, a membrane separation method utilizing the properties of a porous hydrophobic membrane (Patent Literature 1). A description will now be given of the mechanism of the membrane distillation method, with reference to FIG. 1. When one surface of the porous hydrophobic membrane comes into contact with high-temperature primary water (a solution such as sea water) and the other surface of the membrane comes into contact with low-temperature fresh water (pure water), the hydrophobicity of the membrane stops the primary water on the membrane surface and inhibits the permeation of the primary water (as liquid) through the membrane. On the other hand, gas can permeate through the porous membrane. Thus, the permeation of water vapor evaporating from the high-temperature primary water through the membrane and the condensation of the permeated water vapor in the low temperature section allow the separation of only water from the primary water (solution). In other words, in the membrane distillation method, high-temperature supplied water flows onto one side of the membrane and is isolated thereby from a cooling surface provided on the other side of the membrane, whereby a vapor pressure difference due to an occurring temperature difference is used as a driving force for vapor permeation. The problem with the membrane distillation method is that when high-temperature supplied water contains a volatile component, the volatile component easily permeates. Nevertheless, the method exhibits extremely high performance in separation of nonvolatile solutes. Accordingly, for example, highly-pure fresh water can be obtained from sea water containing nonvolatile salt as a main solute.
The basic principle of the membrane distillation method is the same as the evaporation method. However, as compared to the evaporation method, the membrane distillation method has the following advantages:
There is a lot of flexibility in the membrane shape and few limitations on the apparatus shape.
An increase in membrane packing density allows a reduction of the apparatus size.
Due to the mechanism based on the vapor pressure difference, utilization at relatively low temperatures below the boiling point is feasible, and when exhaust heat or water sources having different temperatures can be utilized, there are great energy savings.
Since the solution and permeated water do not come in direct contact, it is almost unnecessary to consider osmotic pressure as in the reverse osmosis method, and the power cost is very low.